In the production of mirrors, a film of silver is formed on one surface of a glass sheet and thereafter a coating or film of copper is formed on top of the silver film, prior to painting the copper film.
A variety of prior art processes, procedures, methods and techniques have been used to form the copper film but each of these prior art processes has substantial drawbacks. One prior art process involves the electrolytic deposition of copper from a suitable solution but this is no longer used for the production of mirrors due to the considerable technical difficulties which resulted in serious flaws and the like.
Accordingly, one of the most commonly used prior art procedures for the deposition of copper on mirrors, silver and the like is the process commonly referred to as the galvanic process and it employs the use of metallic powder as a reducing agent. The major drawback, however, to this galvanic process lies in the difficulty of maintaining a uniform aqueous suspension of the metallic powder which results in serious clogging of the spray guns and apparatus used in the process.
Still another prior art process for the deposition of a copper film on a silver surface or the like is that set forth in U.S. Pat. No. 2,977,244. According to one aspect of this prior art process, a cuprous diammino solution was initially prepared by reacting solid cuprous oxide with an excess of ammonium sulphate in the presence of free ammonia. The reaction according to this prior art process is represented by the following equation: EQU Cu.sub.2 O + (NH.sub.4).sub.2 SO.sub.4 .revreaction. 2Cu(NH.sub.3).sub.2.sup.+ + H.sub.2 O + SO.sub.4 = (1)
however, in this reaction according to the prior art patent, the formation of the cuprous diammino ions is very slow and requires a long time for the cuprous oxide to pass into solution while necessitating the constant stirring of the slurry which is exceptionally inert, especially towards the very weak acidic action of NH.sub.4 +. Still another disadvantage of this patented prior art process is the need to maintain the cuprous solutions used in this process in their reduced forms. Finally, the quality of copper films produced have been found to be inferior to those produced for example by other prior art processes including those discussed above.